The invention relates to a method for optimizing the efficiency of bow and stern screws in ships--particularly ferries--where the rotational speed of the front screw (i.e. bow screw) is in each case less than the rotational speed of the rear screw (i.e. stern screw). In addition, the invention also relates to the associated arrangement for adjusting the rotation speed of the bow screw in those ships having a symmetrical screw arrangement and respectively associated separate drives.
Modern ferries may have a bow screw and a stern screw, which are each driven by separate drives. Such an arrangement avoids having to turn the ship in order to reverse its direction of motion. Such ships with a symmetrical screw arrangement are normally operated with a symmetrical screw arrangement in accordance with an empirically derived characteristic. In particular, this means that the front screw is driven at a rotational speed which is about 10% lower than the rotational speed of the rear screw, depending on the direction of travel, in order as far as possible to avoid in particular any braking effect from the front screw, which cannot be used for thrust.
As is known, the braking effect increases the power consumption of the entire vessel system. A multiplicity of factors which cannot be measured, but which nevertheless influence the optimum rotation speed of the front screw, cannot be taken into account where one merely relies upon previous empirical solutions. Hence, the known procedure remains unsatisfactory in many respects. The efficiency of the entire drive system is, under some circumstances, poor, since changing flow conditions, wave motion and similar external factors are not detected exactly.